The invention relates to the area of advanced biological treatment of waste waters.
The invention also relates to a device for the execution of such a treatment.
U.S. Pat. No. 4,287,062 issued Sept. 1, 1981 to von Nordenskjold describes a biological process using several floating aerator chains spaced apart with respect to each other and moving back and forth which produces a wide-area and uniform aeration and mixing of the waste water containing activated sludge. These plants are characterized, in particular, by the fact that a controlled low loaded activated sludge process can be carried out preferably in earthen basins and without total mechanical mixing of the basin content, i.e., a very wide variety of living conditions for the activated sludge in the basin can be controlled at any time with a comparatively small expenditure of energy.
In the known activated sludge plant the air is introduced into the basin simultaneously through all available bottom aerators for aeration and mixing so that the air is equally distributed along the length of the basin. The movement of the chains accomplishes, among other things, that the activated sludge does not settle at any point of the basin long enough to be damaged.
The bottom aerators, extending as far as the bottom, which pass over the bottom of the tank with a reciprocating movement in the case of the known plant, pick up the activated sludge, circulate it, and also tear it away from the bottom locally by means of the ascending stream of air and water, and thereby also prevent any settled part of the activated sludge from becoming anaerobic by remaining on the bottom sufficiently long to develop negative effects.
Despite the comparatively low specific mixing energy expenditure of this system, it may happen during periods of low load, such as, for example, during night hours, on weekends, etc., that more energy is consumed for the mixing than would actually be necessary or even desirable for the oxygen supply. This leads to undesirable oxygen potentials in the basin, i.e., to an undesired effect on process steps requiring specific oxygen potentials. As used herein, the term "oxidation potential" means the ability of the waste water and its components to perform oxidating or reducing processes.
Waste water treatment plants which are equipped with fixed installed bottom aerators or which use nozzle-like mixing devices belong to the group "high loaded plants" which generally operate at a minimum energy level of 10-15 watts/m.sup.3 basin volume. They also require a horizontal well-constructed basin floor so that the aerators which are supported by the floor will remain at the same horizontal level which is a requirement for an even introduction of air into the basin. There have been several attempts to operate such plants with intermittent airflow through the fixed aerators. Early attempts are described in British Pat. Nos. 1141 of 1915 to Jones and 9989 of 1915 to Naylor, and a later attempt is described in the published European Patent Application of Strassler, No. 0145647. Intermittent operation in such plants is accompanied by various problems. For example, most fixed installed aerators are prone to plugging, especially when they are operated intermittently. When pluggage occurs, then the whole basin has to be emptied to facilitate effective cleaning of the aerators. Thus, this leads to undesired downtime for the plant and high extra maintenance. In addition to the sludge which settles on the aerators when they are shut off, there is also sludge which settles around them. This sludge is difficult to resuspend and part of it remains on the floor and will die, which destroys the whole activated sludge process.
There are several specialized types of plants known to carry out processes which require specific oxygen potentials, e.g., nitrification, denitrification and phosphorus removal. It is known, for example, to operate a separate denitrification basin upstream as well as downstream of the aeration basin in an activated sludge plant. It is also known to have several basins connected in series so that the conditions in each basin can be controlled to permit each of the desired processes to be carried out.
It is also known that such processes can be carried out in circular or oval basins where the waste water is circulated and where, depending on oxygen supply, zones with different oxygen potential are established. To keep the activated sludge in suspension in this latter type of plant, the water must be circulated at a velocity of at least 0.3 meters per second with the help of mixing or pumping devices.